Slow and Steady Dooms the Race

Everyone knows that most sea fishes' reproductive strategy is to spawn millions of eggs at a time. For instance, people in Asia love to eat snakehead fish eggs, and snakehead roes are sold processed into kidney-shaped pairs. One roe contains one to two million eggs. But sharks, which count among the cartilaginous fish, do not follow the same reproductive pattern as the bony fish.

Among the sharks, there are viviparous species (which give birth to live young), oviparous species (which lay eggs), and ovoviviparous species (in which the egg develops within the mother's body but remains separate from it). But whichever their method of reproduction, they all produce very small numbers of young. For instance, the great white shark, which has been listed as a protected species by South Africa and the USA, rarely produces more than 11 pups at a time, and the blue sharks and hammerhead sharks which are caught in huge numbers by Taiwanese fishing boats never produce more than 100 young at once.

But among sharks, these species count as the most prolific breeders. Of all the shark species known to man, more than half only produce two pups at a time. One of them, the tiger shark, only breeds once every two years, so that over its estimated lifespan of 30 years; it cannot produce more than 30 young. But an even more important difference from other fish species is that sharks take up to twelve years to become sexually mature. This is another reason why sharks cannot produce large numbers of offspring.

Dr. Chuang Shou-cheng of the Graduate School of Fisheries Science at Ocean University points out that this is why shark populations have been declining much more rapidly than other fish species. When new shark fishing grounds are discovered by the fishing fleets, they are usually fished out within a short time.

The whale shark--the largest fish species alive on our planet today -- is an example. Taiwan's inshore and offshore waters were home to the world's largest stocks of whale sharks. But then whale shark meat was developed as a new fashionable seafood. Today, less than a decade later, fishermen are hard put to catch a single whale shark, and researchers too can only look at mounted specimens and sigh with regret.

"They hadn't been researched yet, and now they're gone," says Liu Kuang-ming, an associate professor at Ocean University. Just like whales, he says, whale sharks feed mainly on plankton, and have never been known to attack man. Many sea divers who happen on a whale shark will play with it for a while. "If Taiwan brings out a list of sharks which need protection, the whale shark should be at the top."

Biologists believe that sharks' choice of a reproductive strategy similar to that of large mammals, of producing relatively few, well-developed offspring, is linked to their position in the marine food chain.

Most sharks are predators which eat smaller fish, and are at the top of their food chains, so that their numbers are limited anyway. Their role is to maintain the "genetic fitness" of their prey species, thus conserving ecological quality and balance. But mankind's fishing nets have been cast too closely around the sharks, giving them no chance to recover and forcing them to brink of extinction.

The reason why America and Australia have now moved so quickly to protect their sharks has much to do with sharks' slow rate of reproduction. The view that "this in itself is reason enough for Taiwan to implement conservation measures for certain shark species in local waters" is shared by people in Taiwan's fishing industry and conservationist circles alike.